This invention relates to a photosensitive composition, more particularly a photosensitive composition utilized to form a fluorescent screen on the inner surface of the face plate of a color picture tube by a photostickness method or a dry process.
The dry process was developed to replace a conventional slurry process. According to the dry process, a photosensitive composition consisting essentially of an aromatic diazonium salt is used. When locally exposed to light, a film of this composition manifests a difference in the powder accepting capability between a portion exposed to light and a portion not exposed to light.
According to this method, a film of photosensitive substance is coated on the inner surface of a face plate, and the portions of the film at which phosphors of predetermined colors are to be coated are exposed to light through a color selection electrode such as a shadow mask to utilize a photoreaction created therein for the purpose of coating the phosphors. More particularly, a film of a photosensitive composition containing a diazonium salt as a major constituent is coated on the inner surface of the face plate and the portions of the film at which the phosphors are to be coated are exposed to light. Then, zinc chloride formed in the exposed portions by a photoreaction absorbs moisture in the atmosphere to become sticky, so that when a powder of phosphors for emitting desired fluorescent colors is blasted onto the inner surface of the face plate, the phosphor powder will adhere only to the portions of the photosensitive composition film that has become sticky. After performing blasting and sticky coating of respective phosphors of three colors, the surface of the photosensitive composition film is treated with ammonia gas to render the portions of the photosensitive composition film to which the phosphor powder has been adhered to be insoluble in water, thereby fixing these portions. Then, surface remainders are washed away with an organic solvent thus forming a fluorescent screen. When compared with a conventional method of the slurry type in which phosphors of respective colors are formed as photosensitive slurries which are sequentially coated, exposed, washed with water and developed, this dry process is advantageous in that only one coating step of the photosensitive composition film is sufficient, that it is possible to blast the phosphors in the form of a powder, and that surplus phosphor powders that had not adhered can be flown away with an air spray for recovery purpose, thus increasing the efficiency of utilization of the phosphors. Such dry method is disclosed, for example, in Japanese Preliminary Publication of Patent No. 53-126,861, published on Nov. 6, 1978, invented by Saburo Nonagaki et al, and corresponding to U.S. patent application Ser. No. 895,372 filed on Apr. 1, 1978, U.S. Pat. No. 4,273,842 published on June 16, 1981.
Irrespective if the advantage described above, the photosensitivity of the photosensitive position layer formed by the dry process is lower than that of a layer formed by other prior art method. For this reason with the dry process it is necessary to use longer exposure time or larger amount of exposing light than exposing the layer made by the prior art slurry process. Then light diffusion occurs due to reflections between the inner surface of the face plate and the shadow mask or fogs are formed due to a dark reaction.